Use station data for current conditions; persist keys server-side

Station overlay:
- When Ambient Weather station data is present and fresh (<15 min),
  its readings replace the model's current conditions: temp, feels-like,
  humidity, dewpoint, wind speed/direction, gusts, pressure (inHg→hPa),
  UV index, rain today, wind peek summary, and data-panel readouts
- A small "📡 My Station" badge appears in the hero when active
- applyStationOverlay() is called at the end of renderWeather (instant
  if station data already loaded) and again whenever station data
  refreshes (so the display stays live every 60 s)
- Hourly/daily forecasts continue to use Open-Meteo

Server-side key persistence:
- POST /api/ambient-config saves keys to data/ambient-config.json
  (same persistent volume as VAPID and subscriptions)
- GET /api/ambient-config returns {configured: bool} without exposing keys
- /api/ambient checks query params → saved config → env vars in order
- Settings Save button posts keys to server after a successful test;
  confirms "saved for all devices" in the status line
- On page load, if no localStorage keys but server is configured,
  the station loads automatically on any device

app/public/index.html

@@ -64,6 +64,7 @@
           <div id="cur-icon" class="hero-icon"></div>
           <div id="cur-desc" class="hero-desc"></div>
           <div id="cur-precip" class="hero-sub"></div>
+          <div id="station-badge" class="station-badge hidden">📡 <span id="station-badge-name">My Station</span></div>
         </div>
         <div class="hero-center">
           <div class="hero-temp"><span id="cur-temp">--</span><span class="deg">°F</span></div>